Energy minimization calculations on retinal isomers gave quantitative explanations to the disorder in the cyclohexene ring, showing energetics of interconversion consistent with the crystal structure uncertainty and lack of NMR resolution, as well as confirming the known order of stability and extending it to other conformers. The sources of non-planarity and energy differences in the conformers of retinal are due to non-bonded interactions within the molecule. 13C NMR spectra of DDT and analogs have been obtained for correlation with X-ray structural data. The NMR results have been investigated with respect to quantum parameters, Taft and Hammett functions. Energy minimization calculations show the order of stability to be m,p-DDT greater than p,p-DDT greater than o,p-DDT much greater than the l,l,l-tribromo analog. Comparison of their stability energies with reaction rate constants (dehydrohalogenation) and the 13C NMR clearly indicates the large important steric effect due to the chlorine atom in the ortho position. The inhibition, by flavonoids, of aldose reductase, thought to be involved in the formation of sugar cataracts, has been investigated by molecular orbital methods. The data show the efficiency of aldose reductase inhibition depends on the energy of the lowest empty molecular orbital of the flavonoids, and results from a nucleophilic attack on the carbonyl carbon atom.